Hair follicles naturally produce bleaching hydrogen peroxide but this is normally mopped up by the enzyme catalase. As we age, catalase production slows and the hair bleaches itself white or grey. But you have to wait for the hair to grow from the root – once hair emerges from your scalp it is dead. Severe stress could theoretically cause the immune system to cause dark hairs to fall out, leaving just the white ones behind. Nonetheless, there are no scientifically documented cases of this happening after a fright.